Conventionally, sulfonium salt compounds have been used for various applications, such as for a photoacid generator to be used for chemically amplified resist materials. Such a chemically amplified resist material generally contains a resin whose solubility is changed by acid, a photoacid generator, and a solvent. The chemically amplified resist material after being applied is irradiated with radiation such as an electron beam and X-ray within a region of a desired pattern of the applied chemically amplified resist material. Thus, the photoacid generator generates an acid in response to the irradiated radiation, and the generated acid changes the solubility of the resin, which allows a resist pattern for creating an integrated circuit to be formed.
Further, diligently investigation has been made for developing new applications of photoresists using thick film resists or improving conventional products thereof, and there is a demand to form a pattern of such a thick film resist with high accuracy. In order to obtain a pattern of the thick film resist with high accuracy, there is a demand for a photoacid generator having high sensitivity to radiation and high compatibility with other components in the resist material.
I-line radiation at a wavelength of 365 nm is widely used for forming a thick resist pattern using a photoacid generator. One of the reasons for that is availability of light sources such as a high-pressure mercury lamp and a metal halide lamp that allow good emission intensity of i-line light despite its low cost. Recent widespread adoption of LED lamps with an emission wavelength in the i-line region (360 to 390 nm) also can be mentioned. For such reasons, the importance of such a photoacid generator having high responsiveness to i-line light is thought to increase further in the future.
Molecular extinction coefficient (ε) at 365 nm (i-line) is one of indicators for responsiveness to i-line light. As a sulfonium salt compound used as a photoacid generator of this type, an aryldiazonium salt compound (Patent Literature 1), a triarylsulfonium salt compound (Patent Literature 2), and the like have been conventionally proposed. However, aryldiazonium salts and triarylsulfonic acid salts have a maximum absorption wavelength of 300 nm or less, and have a low molecular extinction coefficient (ε) at 365 nm. Therefore, in the case of using a light source such as a high-pressure mercury lamp and a metal halide lamp, there are problems that acid generation efficiency is low, and a resist pattern with high accuracy is difficult to obtain.
On the other hand, an increase in molecular extinction coefficient (ε) at 365 nm does not necessarily lead to an improvement in sensitivity. For example, a sulfonium salt compound into which a thioxanthone skeleton is introduced (Patent Literature 3) absorbs light mostly on the side of the surface on which the resist material is applied because of its excessively high molecular extinction coefficient (ε) at 365 nm (i-line). As a result, the light is not transmitted to a deep portion, and thus the acid generation efficiency rather tends to decrease.
Further, most part of components of a chemically amplified resist material is a solvent. In particular, propylene glycol 1-monomethyl ether 2-acetate (PGMEA) is widely used as the solvent. Therefore, in order to obtain a photoacid generator having high compatibility with other components including such PGMEA, the solubility in PGMEA is very important. Patent Literatures 1 to 3 mentioned above propose BF4−, PF6−, SbF6−, or the like as an anion of the sulfonium salt compound that is used as a photoacid generator of a chemically amplified resist material. However, the sulfonium salt compound containing such an anion generally does not have sufficient solubility in PGMEA.
Therefore, a sulfonium salt compound having a naphthalene ring in a cationic part is, for example, proposed as being useful as a photoacid generator for a chemically amplified resist (see Patent Literatures 4 to 6).